Fast PWM Duty Cycle

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stinkbutt
 
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Re: Fast PWM Duty Cycle

Post by stinkbutt »

I'm sick of theorycraft here. I just ordered a couple of extra inductors from mouser. ($1.50 total - hey big spender.) I'll just drop the 4700 or the 6800 uH inductors into the slot for the 2200 and determine definitively whether the inductor's ah... constipated.

erikd
 
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Re: Fast PWM Duty Cycle

Post by erikd »

Interesting topic. I'm not getting anywhere near 50v out of this boost converter at 90% brightness. I top out at 28v. I believe my converter is running in discontinuous mode. Looking at the output of the converter I see waveforms that look like the right picture in illustration 2 here

http://schmidt-walter.eit.h-da.de/smps_ ... lfe_e.html

I get a good voltage swing to about 45v and then ringing to the end of the cycle. This tends to agrees with pstemari statement below. I only measure a UB+ current of 3.5mA at full brightness. Not sure where the 20mA in the design comes from.

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stinkbutt
 
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Re: Fast PWM Duty Cycle

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Yeah I was going to mention this thread in response to your question at the end of your last thread, but it was kinda off topic. I'm waiting on Mouser or I'd already have a definitive answer on this. A 6800 uH inductor ought to clear things up, one way or the other.

As for the 20 mA, that's from Adafruit direct, but in light of this 3.5 mA measurement, I suspect the MAX6921 driver's drawing less because it's using a blanking interval to reduce ghosting. This current starves the boost converter, we go into discontinuous mode, and the voltage rises. We can't really do anything about this short of using a bigger inductor, because we're running the fast PWM at max right now already.

I am hopeful I'll be able to confirm this with a bigger inductor, which should see the voltage out of the boost converter drop closer to continuous mode, but if your reading of 3.5 mA is to be believed, even 6800uH might not be enough on it's own. I guess it's a good thing Mouser's taking forever to get me my stuff - Time to edit the order...

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Re: Fast PWM Duty Cycle

Post by pstemari »

Hmm, time to go look at those formulae again. I would have instinctively said bigger inductor == more energy == higher voltage, but inductors are just generally freaky. My own damn fault for dropping EE40 all those years ago, I suppose.

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Re: Fast PWM Duty Cycle

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stinkbutt wrote:Yeah I was going to mention this thread in response to your question at the end of your last thread, but it was kinda off topic. I'm waiting on Mouser or I'd already have a definitive answer on this. A 6800 uH inductor ought to clear things up, one way or the other.

As for the 20 mA, that's from Adafruit direct, but in light of this 3.5 mA measurement, I suspect the MAX6921 driver's drawing less because it's using a blanking interval to reduce ghosting. This current starves the boost converter, we go into discontinuous mode, and the voltage rises. We can't really do anything about this short of using a bigger inductor, because we're running the fast PWM at max right now already.

I am hopeful I'll be able to confirm this with a bigger inductor, which should see the voltage out of the boost converter drop closer to continuous mode, but if your reading of 3.5 mA is to be believed, even 6800uH might not be enough on it's own. I guess it's a good thing Mouser's taking forever to get me my stuff - Time to edit the order...
The 10 mH (10,000 uH) inductor arrived last week and I just dropped it in. Inserting it in drops the output voltage of the boost converter from 26V to 16V. That pretty much settles that question. The boost converter is operating in discontinuous mode, at least with the 2200 uH inductor. I really can't say if it's still in discontinuous mode with the 10 mH inductor. I only soldered it in for a few minutes to test the voltages, and then I pulled it out. You cannot fit a big-ass inductor into the clock. Not only was I unable to get it flush against the board, but it was so big it abutted up against the tube preventing it from seating properly in the right-angle connectors.

So I basically set fire to a couple of dollars (admittedly a risibly small sum) to answer this question of the boost converter is operating in. I guess in retrospect the designers probably just decided that although the converter wasn't behaving the way they expected it to, it was close enough, and I'm not sure how you jam a super-big inductor into that form factor. As it is the 10 mH inductor's the tallest part on the board even if it were flush, which it couldn't be without a larger board footprint.

erikd
 
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Re: Fast PWM Duty Cycle

Post by erikd »

I played around with some inductor values and discovered the same as you. A 10mH inductor dropped my 30vdc to around 16. I settled on a 820uh inductor for my clock. That gives me a max voltage of about 48 vdc at max brightness.

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stinkbutt
 
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Re: Fast PWM Duty Cycle

Post by stinkbutt »

erikd wrote:I played around with some inductor values and discovered the same as you. A 10mH inductor dropped my 30vdc to around 16. I settled on a 820uh inductor for my clock. That gives me a max voltage of about 48 vdc at max brightness.
To confirm, do you mean 8200 uH or 820 uH? Are you going for a higher voltage than stock?

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Re: Fast PWM Duty Cycle

Post by erikd »

820uh, lower value then what's called for in the schematic. My stock high voltage was only about 30vdc at full brightness. I could find no reason for this other then my high voltage current was much lower then indicated by adafruit's design notes, only 3mA as opposed to 20mA. I wanted to get the high voltage back to around 50vdc. Since I'm more hardware than software it was easier for me to try different values for the inductor to get my HV back to where it was designed to be rather then try to understand that math (with didn't include HV load current) to play with duty cycles in software.

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